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Conduction System of the Heart

Faisal I. Mohammed, MD, PhD

Yanal A. Shafagoj MD, PhD

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Objectives

List the parts that comprise the conduction system

Explain the mechanism of slow response action

potential (pacemaker potential)

Point out the regulation of the conduction system

potential by Autonomic Nerves

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Heart Physiology: Sequence of Excitation

Conducting System of Heart

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Intrinsic Cardiac Conduction System

Approximately 1% of cardiac muscle cells are

autorhythmic rather than contractile

70-80/min

40-60/min

15-40/min

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Intrinsic Conduction System

Function: initiate & distribute impulses so heart depolarizes & contracts in orderly manner from atria to ventricles.

SA node

AV node

Bundle of His

Bundle Branches

Purkinje fibers

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Sinus Node

Specialized cardiac muscle connected to atrial

muscle.

Acts as pacemaker because membrane leaks Na+ and

membrane potential is -55 to -60mV

When membrane potential reaches -40 mV, slow

Ca++ channels open causing action potential.

After 100-150 msec Ca++ channels close and

K+channels open more thus returning membrane

potential to -55mV.

Fast Response Action Potential of Contractile Cardiac Muscle Cell

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Pacemaker and Action Potentials of the Heart

Slow Response Action Potential (Pacemaker

Potential)

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Intrinsic rate and speed of conduction of the components of the system

SA node 60-80 action potential /min (Pacemaker)

AV node 40-60 action potential /min

Purkinje 15-40 action potential /min

Conduction Speed

SA node: slow speed of conduction

Ventricular and Atrial muscle: Moderate speed

AV node: slowest speed of conduction

Purkinje fibers: Fastest speed of conduction

Ectopic Pacemaker- Abnormal site of pacemaker

Extrinsic Innervation of the Heart

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Autonomic Innervation of the Heart

Pacemaker Function

Sympathetic – increases heart rate by Ca+2 & If channel (net

Na+) flow

Parasympathetic – decreases rate by K+ efflux & Ca+2 influx

Autonomic neurotransmitters affect ion flow to change rate

What part of the graph is not changed by autonomic influences?

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Regulation of the heart beat

Sympathetic from the cardiac plexus supplies all parts of the heart (atria, ventricle and all parts of the conduction system)

Parasympathetic from Vagus nerves supply mainly the atria, SA and AV nodes, very little supply to ventricles

Sympathetic: increase the permeability of the cardiac cells to Na+ and Ca++ i.e Positive Chronotropic and positive Inotropic action

Parasympathetic: Increase the permeability of the cardiac cells to K+ and decrease its permeability to Na+ and Ca++

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Sinus Node is Cardiac Pacemaker

Normal rate of discharge in sinus node is

70-80/min.; A-V node - 40-60/min.;

Purkinje fibers - 15-40/min.

Sinus node is pacemaker because of its

faster discharge rate

Intrinsic rate of subsequent parts is

suppressed by “Overdrive suppression”

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Ectopic Pacemaker

This is a portion of the heart with a

more rapid discharge than the sinus

node.

Also occurs when transmission from

sinus node to A-V node is blocked (A-V

block).

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Parasympathetic Effects on Heart Rate

Parasympathetic (vagal) nerves, which release

acetylcholine at their endings, innervate S-A node

and A-V junctional fibers proximal to A-V node.

Causes hyperpolarization because of increased K+

permeability in response to acetylcholine.

This causes decreased transmission of impulses

maybe temporarily stopping heart rate.

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Sympathetic Effects on Heart Rate

Releases norepinephrine at sympathetic

ending

Causes increased sinus node discharge

(Chronotropic effect)

Increases rate of conduction of impulse

(Dromotropic effect)

Increases force of contraction in atria and

ventricles (Inotropic effect)

Thank You